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Series Chillers

Does anyone have any general information on the sequencing / control of a series chiller plant confuration. This is an unusual configuration here and I have only ever seen it once previously a long time ago and can't remember anything at all about the control sequences. The chillers are equal size Trane Centravac's with both load limiting and setpoint control capability via the BMS.

I have a job installed a year ago, same mess. They had to put them in series to get the temp drop they wanted. There was no control, one set to take the bulk of the load coming back, leaving water setpoint of 48, second one leaving setpoint of 40. After a year, the last chiller has 2000 more hours than the first as you can figure. I created a sequnce to rotate to even up runtime, it's pretty complicated and I don't have it on this computer. get back to you.

"Yeah I can figure out whats wrong with it, but you were here first and there isn't room for two, plus it's cold up here, I'll be in the van"

I have a job installed a year ago, same mess. They had to put them in series to get the temp drop they wanted. There was no control, one set to take the bulk of the load coming back, leaving water setpoint of 48, second one leaving setpoint of 40. After a year, the last chiller has 2000 more hours than the first as you can figure. I created a sequnce to rotate to even up runtime, it's pretty complicated and I don't have it on this computer. get back to you.

In this system, was the condenser lines piped in series or parallel? Also, by alternating the chiller operation in this manner, doesn't the chiller first in line act like a heat exchanger, thereby forcing the second chiller to do more work?

I've never seen this application before, i've always seen it in parrallel; but it seems like the chiller that's first in line should always be on and the second on come on only if the first chiller can't maintain the setpoint.

I worked on a system like that a couple years ago in a proccess plant with another guy in our company that has allot of control and chiller expirience. In our application the chillers were seriesed to provide a high differential temperature at full load but one chiller could handle the load at partial load. Heres what we did. We programmed the system to do a lead/lag/fail routine with some exceptions to equalize load on the chillers and run time. As said one chiller will carry the load and make the required leaving water temp at partial loads so that was a standard lead/lag/fail routine to equalize run time. We gave the chiller running what the actuall leaving water stpt was. When the leaving water temp rose above the setpoint when one chiller could no longer carry the load we would stage the second chiller on. At this point we would give the chiller on the leaving water side the final water temp stpt (lets say 40). We had a sensor on the return loop so we new what the actual return temp was. Say we had a 55 degree return so we actually had a 15 degree delta. We would set the upstream chillers setpoint at 47.5(half the system delta). This would make each chiller carry a 7.5 delta or approximately half the load. As the delta changed so would the upstream chillers stpt. ( We did limit how often it would recive an updated setpoint though) When the delta shrank back down and stayed there for a length of time where one chiller could carry the load again we would turn the second chiller off and go back to our lead/lag/fail routine. This was all done with one small stand alone controller on the wall interfacing with the chillers with simple analog and binary inputs and outputs. The sequence will work no matter how you implement the chiller interface.

In this system, was the condenser lines piped in series or parallel? Also, by alternating the chiller operation in this manner, doesn't the chiller first in line act like a heat exchanger, thereby forcing the second chiller to do more work?

I've never seen this application before, i've always seen it in parrallel; but it seems like the chiller that's first in line should always be on and the second on come on only if the first chiller can't maintain the setpoint.

What are your thoughts?

They are air cooled screws setup for low temp operation, VFD, multiple compressors. As I remember the design EWT is 65 and LWT 40. The entering chiller is 30 tons larger.

Without controls, the chillers operate independent with leaving set points of 48 and 40. You can see the potential problem, the leaving chiller would always run, under constant load and the entering chiller would cycle up/down on/off if it's leaving temp came close to 48, yet it's brother was humming along under a load. The entering chiller is larger so it can handle the entire load under low ambient conditions allowing the smaller one be disabled to even up runtime, otherwise we just change the setpoints to balance load.

We pretty much programmed similar to what was referred to below. We look at VFD output, number of compressors running, OA temp, operational codes, along with the temps. We also have complete integration to the chillers.

"Yeah I can figure out whats wrong with it, but you were here first and there isn't room for two, plus it's cold up here, I'll be in the van"

Thanks

Thanks for the info, the reference http://tristate.apogee.net/cool/cxfrc.asp
helps. I am going to get the original specification soon so hopefully I can get to the bottom of exactly why series machines were specified.

Regards and have a good Xmas (19 hours after the right end of the planet does!)